Because of their strength and heat and chemical resistance, polyester fibers and films are an integral component in numerous consumer products manufactured worldwide. The overwhelming majority of the commercial polyester used for polyester fibers and films is polyethylene terephthalate (PET) polyester. Because PET forms a lightweight and shatterproof product, one popular use for PET is as a resin for beverage bottles.
Prior to 1965, the only feasible method of producing PET polyester was to react dimethyl terephthalate (DMT) with ethylene glycol in a catalyzed ester interchange reaction to form bis(2-hydroxyethyl)terephthalate monomer and methanol. The monomer is then polymerized through polycondensation to produce polyethylene terephthalate. Because purer forms of terephthalic acid (TA) have become increasingly available, TA has become an acceptable alternative to DMT as a starting material for the production of polyethylene terephthalate. In a reaction similar to that between DMT and ethylene glycol, terephthalic acid and ethylene glycol react in a generally uncatalyzed esterification reaction to yield low molecular weight oligomers and water. As with DMT, the monomer is subsequently polymerized by polycondensation to form PET polyester. The resulting PET polymer is substantially identical to the PET polymer resulting from DMT with the possible exception of some of the end groups.
The conventional method of carrying out the formation of PET polyester was in a batch process. In the conventional batch process, the product of the ester interchange or esterification reaction was formed in one vessel and then transferred to a second vessel for polymerization. Generally, the second vessel was agitated and the polymerization reaction continued until the power used by the agitator reached a level indicating that the polyester melt had achieved the desired intrinsic viscosity and therefore, the desired molecular weight. Eventually, the polymerization reaction and later the esterification and ester interchange reactions were carried out as continuous reactions. The continuous production of PET results in greater throughput and has since been adopted in most large-scale manufacturing facilities.
When the polymerization process has been completed, the resulting polymer melt is typically extruded and pelletized for convenient storage and transport before being worked up into specific polyester articles such as filament or bottles or other items. Such steps are also typically labelled as "polyester processing" but refer of course to later working of the finished polyester rather than to the chemical processing steps used to form the polyester in the first place.
In both the batch and the continuous processes, a high activity catalyst is often employed to increase the rate of polymerization thus increasing the throughput of the resulting PET polyester. The high activity catalysts which are used in the polymerization of PET polyester can be basic, neutral or acidic, and are often metal catalysts. Primarily, the traditional polymerization catalysts used in the formation of PET from both TA and DMT contain antimony and the most common of the antimony-containing catalysts is antimony trioxide, Sb.sub.2 O.sub.3. Although polymerization catalysts such as antimony trioxide result in the increased production of PET, these same polymerization catalysts will eventually begin to catalyze or encourage the degradation of the polymer formed in the condensation reaction. Such degradation of the PET polymer results in the formation of acetaldehyde and the discoloration or yellowing of the PET polyester.
Additionally the availability of newer "hotter" catalysts that can significantly increase throughput has generated a corresponding need for better stabilization of the resulting polyester. U.S. Pat. No. 5,008,230 to Nichols is exemplary of such an improved catalyst.
In an attempt to reduce the degradation and discoloration of the PET polyester, stabilizing compounds are used to sequester ("cool") the catalyst thus reducing its effectiveness. The most commonly used stabilizers contain phosphorous, typically in the form of phosphates and phosphites. The phosphorous-containing stabilizers were first employed in batch processes to prevent degradation and discoloration of the PET polyester. For example, U.S. Pat. No. 4,122,063 to Alexander et al. describes the addition of triphenyl phosphates and 1,2-epoxy-3-phenoxypropane to stabilize the antimony trioxide catalyst in the post-reaction PET polyester. U.S. Pat. No. 4,385,145 to Horn, Jr. describes the addition of pentaerythritol diphosphite esters to poly(alkylene terephthalate) in a batch process to stabilize the catalyst in the post-reaction polyester thus preventing degradation and discoloration of the polyester. U.S. Pat. No. 4,401,804 to Wooten et al. describes the addition of phosphate, phosphonate and phosphite compounds to stabilize post-reaction poly(1,4-cyclohexylenedimethyl terephthalate) polyester in a batch process. U.S. Pat. No. 4,680,371 to Rosenfeld et al. describes the addition of phosphites to stabilize the basic catalysts used in the polymerization of certain aromatic polyesters. U.S. Pat. Nos. 4,824,895 and 4,829,113, also to Rosenfeld et al., describe the addition of stabilizers containing phosphorous, oxygen, sulfur, or fluorine to stabilize basic catalysts in the same aromatic polyester compounds.
Although adding a stabilizer to the polymer melt in a batch reactor is a relatively simple process, numerous problems arise if the stabilizers are added in the continuous production of PET. For instance, if the stabilizer is added after the polymerization process, i.e., during polymer processing, it may not fully blend with the polymer melt and may not prevent degradation and discoloration of the polyester. Furthermore, addition of the stabilizer during polymer processing is inconvenient and does not provide economy of scale.
One solution provided by the previously mentioned Rosenfeld patents is to add the stabilizer to the molten monomers early in the polymerization process, even before the polycondensation reaction, to prevent neutralization of the basic catalyst and discoloration of the aromatic polyesters. Nevertheless, the Rosenfeld patents are directed to the use of basic catalysts in the formation of specific aromatic polyesters and are neither directed to PET polyester nor do they specifically address the continuous formation of polyesters using Lewis acid polymerization catalysts.
Although early addition of the stabilizer prevents discoloration and degradation of the polyester, it also unfortunately results in reduced production or throughput of the polyester, i.e., reduced molecular weight of the polyester caused by a decrease in the reaction rate for the polycondensation reaction. Furthermore, the stabilizer is typically dissolved in ethylene glycol, the addition of which further slows the polymerization process. Therefore, early addition of the stabilizer in the polymerization process tends to force an undesired choice between production throughput and thermal stability of the polymer. As used herein, "thermal stability" refers to a low rate of acetaldehyde generation, low discoloration, and retention of molecular weight following subsequent heat treatment or other processing.
Therefore, in order to increase the throughput of PET polyester while limiting discoloration and degradation of the polyester, a continuous process which stabilizes the PET produced is needed in the art.